Johannesburg - Before 1994, Category 5 (CAT5) tropical cyclones - the strongest category of these storms - did not materialise in the South Indian Ocean.
The first tropical cyclone in the South Indian Ocean to intensify to CAT5 status did so that year.
Over the past 24 years, these destructive storms have become more frequent, and pose “catastrophic” threats of damage for South Africa, warns climate change researcher Dr Jennifer Fitchett in a new paper.
Fitchett, a senior lecturer in geography at Wits University, says her research has shown how CAT5 storms in the South Indian Ocean have become more common as the ocean warms from global climate change.
“Category 5 tropical cyclones, the strongest category of storms, have only recently emerged in the South Indian Ocean,” writes Fitchett in her paper, Recent emergence of CAT5 tropical cyclones in the South Indian Ocean, published in the latest edition of the South African Journal of Science.
“Since 1989, their frequency of occurrence has increased. This increase poses a heightened risk of storm damage for the South Indian Ocean Island States and the countries of the southern African subcontinent as a result of the strong winds, heavy rainfall and storm surges associated with these storms, and the large radial extent at category five strength.”
This is significant, she says, for the forecasting of tropical cyclone landfall and the anticipation of storm damage for developing economies that characterise the region, she writes.
Although an increase in tropical cyclone intensity is frequently projected under global climate change scenarios, the dynamics for the South Indian Ocean are poorly understood.
“Notable are early results indicating an increased frequency and poleward migration of these CAT5 storms, concurrent with a poleward migration in the position of the 26.5°C, 28°C and 29°C sea surface temperature isotherms in the South Indian Ocean."
The increase in the frequency of occurrence of CAT5 storms, particularly for a region that did not previously experience very high intensity tropical cyclones, “demonstrates the manifestation of the sea surface temperature warming on a tropical cyclone system”, she writes.
Her results provide a concerning outlook for the South Indian ocean. “The region comprises a number of economically developing countries and small island states, which cannot afford large capital investment in infrastructural adaptation measures to mitigate against the threats of tropical cyclones.”
Unlike the US, which experiences numerous tropical cyclones making landfall in any given year, only 5% of the nine tropical cyclones that form in the western half of the south Indian Ocean basin make landfill in any given year.
“However, all of the storms that make landfall have devastating impacts on the livelihoods, habitat, economy and natural environment of the country affected.
“Thus the threat of increasing proportions of the highest intensity tropical cyclones is potentially devastating.
“Furthermore, the poleward trajectory of these storms indicated by the 12 CAT5 tropical cyclones that have been recorded thus far, and global studies indicating a poleward trajectory in the lifetime maximum intensity of tropical storms, pose a heightened threat for SA.”
Although the SA coastline is protected from tropical cyclones by Madagascar, “this southward trajectory has the potential to heighten the proportion of storms tracking south of this island nation, which currently takes the brunt of tropical cyclones in the South Indian ocean”.
For South Africa, as one of the most economically important countries in sub-Saharan Africa, even infrequent storms pose the treat of catastrophic damage.
“High-intensity storms would not only increase the potential for damage through the heightened wind speeds and rainfall but storms of higher intensity additionally have a wider storm radius, increasing the region of damage on landfall.”